A distillation method is common as a method for separating and purifying an acryl monomer such as a (meth)acrylic acid compound. In recent years, a high performance packing material has been developed for the purpose of e.g. improving the separation efficiency by distillation or increasing the amount to be treated and has been practically employed in distillation columns in various processes. However, a (meth)acrylic acid compound is extremely polymerizable, and formation of a polymer in a distillation column has been a serious problem in a conventional tray type distillation column, particularly in a high performance packed column.
Heretofore, it is known to improve the tray structure (e.g. JP-A-2000-300903) as a method for preventing formation of a polymer of a (meth)acrylic acid compound. Further, a method of using a special polymerization inhibitor (e.g. JP-A-7-53449) has been proposed. However, it has been difficult to conduct a continuous operation for a long period of time, and periodical inspection, washing, repair, etc. have been required, which require suspension of the operation.
As a method for such washing or repair, a method has been proposed which comprises washing with a basic solution of e.g. sodium hydroxide or potassium hydroxide, followed by washing with a solvent (particularly preferably water) (e.g. JP-A-2000-319223). In a case where a (meth)acrylic acid compound is thus treated by a distillation column, it is common to finally wash the interior with water for the purpose of safety, in order to inspect the distillation column during the suspension.
At the time of resuming the operation after completion of the inspection, etc. of the distillation column washed with water as mentioned above, if such water remains in the system, the time after the resumption of the operation until the distillation column becomes stabilized in a steady condition, will be prolonged, and a non-steady operational composition state has to be continued. It has been found that continuation of this non-steady state brings about polymerization of the (meth)acrylic acid compound which is a polymerizable substance.
Thus, the object of the present invention is to provide a process for washing a distillation column for separating and purifying a crude (meth)acrylic acid compound. Particularly, in a process for producing a (meth)acrylic acid compound, it is to provide a method for cleaning the distillation column efficiently in a short time and recovering a valuable substance, by utilizing a substance used in a process before and after the distillation column.